first since you mentioned your friend is a teenager and under 21, he
<U>has</U> to take the MSF course (
www.msf-usa.org) After he takes that course he has to get his M1 certification. Having a normal C certification wont let you qualify for the motorcycle, you got to get the M1. When you pass the MSF course you get a dmv waiver that lets you take the written test and get the M1 cert.
Now as for the bike, an older 500cc bike will do. Do not go over 500cc cause insurance will spike if its a sports bike. A cruiser you can go upto 750cc. I would suggest a Ninja500R or Suzuki GS500 (the one i have - look signature). The Ninja500R will have a slightly higher insurance cause its got plastic. Also since your buying used you got to make sure the Ninja is well maintained, (valve adjustments are done regularly - most GS500 riders do it regularly).
I must mention though having a bike is sweet, its not practical for carrying stuff etc etc. I would suggest getting a decent helmet (check
www.motorcycle-usa.com and under superstore), something with DOT and Snell M2000 rating on it. I prefer the Vega, its cheap and its good, but even HJC has one at that range. You will need gloves and a jacket to keep the wind off you. Preferable leather if not cordura. You will also need some over the ankle boots. Yes you can ride with sneakers but it doesnt provide any ankle protection.
If you plan to ride in the rain you will need a rain suit (about 50 bucks or so).
Riding a bike is no ball game, personally i would suggest your friend getting an older car (may be not a honda but a american car like a buick or something - insurance is cheap on american cars) and then later when he has some driving experience to get a bike. Without driving experience riding a bike is harder, cause there are lots more decisions you got to make when your a rider.
Also when shopping for insurance shop around, check with all companies, geico is who I am going with right now. but there are other companies.